Method for polishing semiconductor wafers

ABSTRACT

A method for polishing a plurality of semiconductor wafers includes providing a polishing pad containing an abrasive substance bonded in the polishing pad; providing an alkaline polishing agent at a volumetric flowrate greater than or equal to 5 liters/min.; polishing the plurality of semiconductor wafers using the polishing pad; and circulating the polishing agent in a polishing agent circuit during the polishing.

CROSS REFERENCE TO PRIOR APPLICATIONS

Priority is claimed to German Patent Application No. DE 10 2009 047926.0, filed Oct. 1, 2009. The entire disclosure of which isincorporated by reference herein.

FIELD

The invention relates to a method for polishing semiconductor wafers.

BACKGROUND

Semiconductor wafers, in particular silicon wafers, are used for thefabrication of large scale integrated electronic components such as e.g.microprocessors or memory chips. In this case, stringent requirementsare made in particular of the flatness of the front sides of the siliconwafers on which the electronic components are produced. This isnecessary in order to minimize problems in the course of the exposure ofthe silicon wafers (lithography) and in the course of intermediatepolishing processes (“Chemical Mechanical Polishing”, CMP) during thefabrication of the components.

Polishing the surfaces of semiconductor wafers pursues the goal ofremoving material from the surfaces of the semiconductor wafers in orderto form a planar surface that is as uniform as possible. As a result, anundesirable surface topography and surface defects such as roughsurfaces, crystal lattice damage or scratches can be removed and uniformsurfaces can be made available for the subsequent further processing.

Therefore, after grinding, cleaning and etching steps in accordance withthe prior art, the surface of the semiconductor wafers is smoothed byremoval polishing.

In the case of double-side polishing (DSP), semiconductor wafers areintroduced loosely into a so-called carrier and are polished on thefront and rear sides simultaneously in a manner “floating freely”between an upper and a lower polishing plate covered with a polishingpad, with the aid of a polishing sol. The support parts supply acontrollable pressure that presses the semiconductor wafer against thepolishing support.

In the prior art, the polishing is carried out by relative movementbetween wafer and polishing pad under pressure and with a polishingagent (slurry) being supplied. The polishing agent is colloidallydispersed silica sol, for example. The polishing pad contains noabrasive. The interplay between mechanically abrasive action of thesilica sol and chemical attack of the alkaline polishing agent thenbrings about the material removal that leads to the smoothing of thewafer surface.

One exemplary embodiment of DSP of silicon wafers is disclosed inUS2003054650A. A suitable apparatus for such DSP polishing is presentedin DE 100 07 390 A1.

In the case of CMP polishing, by contrast, only the front side ispolished, for example by means of a soft polishing pad.

In conventional polishing agent preparation methods, the polishing agentconstituents are procured as concentrated solutions and/or as solids.The solids, however, are generally dissolved with ultrapure water priorto use in separate preparation stations. These polishing agentconstituents and ultrapure water are processed further in centralpreparation stations to form the finished polishing agent or to formpartial mixtures. The solutions thus produced are usually storedtemporarily in suitable containers. The ready to use mixture is conveyedto the consumers via a ring line. The polishing agent is then drawn offfor the processing step via a branch line. The used polishing agent issubsequently discarded.

Conventionally, alongside the colloidally dispersed silica sol, analkaline buffer solution is also used, e.g. K2CO3 or KOH solution.

SUMMARY OF THE INVENTION

In the use known from the prior art, the alkaline buffer solutions aresuspected of causing metallic contamination on semiconductor wafers.

In an aspect of the invention this is avoided.

The invention relates to a method for polishing semiconductor wafersusing a polishing pad containing an abrasive substance bonded in thepolishing pad, and with an alkaline polishing agent being supplied,wherein a volumetric flow rate of the polishing agent is greater than orequal to 5 liters/min and the polishing agent is circulated in apolishing agent circuit during polishing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flowchart of the method of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a method 100 for polishing a plurality of semiconductorwafers. A polishing pad is provided containing an abrasive substancebonded in the polishing pad (10). An alkaline polishing agent isprovided at a volumetric flowrate greater than or equal to 5 liters/min.(20). The plurality of semiconductor wafers are polished using thepolishing pad (30). The polishing agent is circulated in a polishingagent circuit during the polishing (40).

The alkaline polishing agent is preferably ultrapure water or deionizedwater containing compounds such as sodium carbonate (Na2CO3), potassiumcarbonate (K2CO3), sodium hydroxide (NaOH), potassium hydroxide (KOH),ammonium hydroxide (NH4OH), tetramethylammonium hydroxide (TMAH) or anydesired mixtures of these compounds.

The use of TMAH is especially preferred.

The pH value of the polishing agent preferably lies in a range of 10 to12.

The proportion of the stated compounds in the polishing agent solutionis preferably 0.01 to 10% by weight, particularly preferably from 0.01to 0.2% by weight.

The polishing agent solution can furthermore contain one or more furtheradditives, for example surface-active additives such as wetting agentsand surfactants, stabilizers acting as protective colloids,preservatives, biocides, alcohols and complexing agents.

In this case, the polishing agent is free of solids.

The polishing agent can also be a slurry containing abrasives.

The proportion of the abrasive substance in the polishing agent slurryis preferably 0.25 to 20% by weight, particularly preferably 0.25 to 1%by weight.

The size distribution of the abrasive substance particles is preferablymarkedly monomodal.

The average particle size is 5 to 300 nm, particularly preferably 5 to50 nm.

The abrasive substance is preferably composed of one or more of theoxides of the elements aluminum, cerium or silicon.

A polishing agent slurry containing colloidally dispersed silica isparticularly preferred.

The silicon wafers can either be pressed with the aid of a polishinghead with the side surface to be polished against the polishing padlying on a polishing plate and be polished in this way or alternativelybe subjected to free floating double-side polishing—in a manner guidedby carriers.

In the case of polishing with a polishing head, the latter preferablyalso includes a retainer ring that laterally encloses the substrate andprevents it from slipping from the polishing head during polishing.

In modern polishing heads, that side surface of the silicon wafer whichis remote from the polishing pad bears on an elastic membrane thattransmits the polishing pressure exerted. The membrane is part of apossibly subdivided chamber system that forms a gas or liquid cushion.

However, polishing heads are also in use in which an elastic support(“backing pad”) is used instead of a membrane.

The substrate is polished with the polishing agent being suppliedbetween the substrate and the polishing pad and with rotation of thepolishing head and of the polishing plate.

In this case, the polishing head can additionally also be movedtranslationally over the polishing pad, whereby more comprehensiveutilization of the polishing pad area is achieved.

The method according to the invention can be carried out equally onsingle-plate and multi-plate polishing machines; in the form of both asingle-side and a double-side polishing process.

Preference is given to the use of multi-plate polishing machines havingpreferably two, especially preferably three, polishing plates andpolishing heads.

In the method according to the invention, a polishing pad is used whichcontains an abrasive substance bonded in the polishing pad (FAP or FApad).

Suitable abrasive substances comprise, for example, particles of oxidesof the elements cerium, aluminum, silicon, zirconium and particles ofhard substances such as silicon carbide, boron nitride and diamond.

Particularly suitable polishing pads have a surface topography shaped byreplicated microstructures. These microstructures (posts) have, forexample, the form of pillars having a cylindrical or polygonal crosssection or the form of pyramids or truncated pyramids.

More detailed descriptions of such polishing pads are contained in WO92/13680 A1 and US 2005/227590 A1, for example.

The grain sizes of the FAP polishing pads used are preferably greaterthan or equal to 0.1 μm and less than or equal to 1.0 μm.

In principle, the polishing agent is preferably led past the polishingmachine and circulated by means of ring lines (polishing agent circuit).

The media circulated in the polishing agent circuit are preferablycooled. The cooling is preferably effected by heat exchanger elementsincorporated into the polishing agent circuit.

In the vicinity of the polishing machine, the polishing agent ispreferably drawn off by means of a distributor unit and then introducedbetween polishing pad and semiconductor wafer.

The flow rates of the polishing media used are preferably set by meansof measuring and regulating circuits as follows: the flow of thepolishing media is set by means of flow controllers with actuating orneedle valves or by means of metering pumps. The valves are set by meansof a change in the line diameter. The flow rates are measured by meansof impeller-type flow meters, for example. It goes without saying thatautomatic control by means of suitable software is possible andpreferred.

The media regulated in this way, which can comprise abrasive polishingagent components, solutions and water, can be led into a mixing unit(e.g. to a pipe with static mixing elements), in which the polishingagent is mixed.

The polishing agent mixed in this way can then be led either firstlyinto a holding station or else directly into a holding container servingas a recycling holding container and as a container for possibly toppingup the polishing agent.

From the holding container, the polishing agent is preferably conveyed,via a ring line, to one or more consumers and, by means of a branchline, to the polishing machine.

Polishing agent which has not been used flows back to the holdingcontainer and is conveyed to the consumer(s) again via the ring line.

The polishing agent consumed during polishing is collected by means of asuitable collection system and returned to the holding container via aline system. Said line system preferably provides a discharge, that isto say that, preferably, part of the consumed polishing agent is removedand not returned to the holding container. A corresponding amount ofnew, unconsumed polishing agent is then fed to the holding container.

Consequently, a specific portion of the polishing agent already used ispreferably always replaced by new polishing agent.

Preferably, the alkaline component of the polishing agent is used fortopping up the same.

Preferably, the pH value of the polishing agent in the holding containeris always monitored by means of online analysis. If necessary, bydischarging consumed polishing agent and topping up the polishing agentin the holding container, the pH value is correspondingly corrected,preferably by feeding in an alkaline solution.

The filling level of the holding container is preferably always at aspecific minimum level. This is ensured by corresponding preparationwith fresh polishing agent.

In the method according to the invention, a defined amount orconcentration of polishing agent is preferably collected in a holdingcontainer and then reused, a defined amount of consumed polishing agentbeing replaced by fresh polishing agent. This takes place in such amanner that, firstly, fresh polishing agent is produced and, after ithas been used once, it is guided into a holding container. If thefilling level in the holding container reaches its highest level, theprocessing process is changed over completely to the polishing agentwhich has already been used once and is located in the collectioncontainer. Depending on the particular application, the filling level inthe holding container decreases as a result of media losses/rinsinglosses, clarification rate, etc. When a specific filling level in theholding container is reached or after a predefined duration of use orafter a specific number of wafers, the polishing agent which has alreadybeen used one or more times is topped up by the addition of freshpolishing agent from a supply container.

Suitable for carrying out the method is a polishing installation forsemiconductor wafers which has means which are able to replace polishingagent within defined limits of amount and concentration. A polishinginstallation of this type preferably has at least one holding containerfor polishing agent which has already been used and a container forfresh polishing agent, and also means for discharging consumed polishingagent and adding fresh polishing agent.

One preferred polishing installation for semiconductor wafers is onewhich has a regulator, the polishing agent being replenished on thebasis of the chemical consumption. This is carried out by means of anapparatus which determines the chemical consumption during the polishingmethod in the outgoing flow of polishing agent and then suppliescorrespondingly fresh polishing agent or specific polishing agentcomponents. The chemical consumption is determined by means of anelectrode, for example.

A further preferred polishing installation for semiconductor wafers hasmeans which discharge and replace a defined amount or concentration ofpolishing agent. These means are, for example, commercially availablemeasurement systems and pumps.

A further preferred polishing installation for semiconductor wafers hasmeans which collect a defined amount or concentration of polishing agentin a holding container for reuse and means which replace consumedpolishing agent by a defined amount of fresh polishing agent.

The advantages of the present invention reside in the fact that thecomposition of the polishing agents remains stable even in the case ofregenerated polishing agents.

The polishing abrasion rate remains virtually constant and there is noneed to readjust the polishing time, in which case the polishing timesalso remain constant and a better quality, for example with respect tothe desired thickness, is achieved.

Use of relatively high volumetric flow rates of greater than or equal to5 liters/min is provided according to the invention. Preferably, thevolumetric flow rate of the polishing agent is 5-10 liters/min,particularly preferably 5-9 liters/min, and especially preferably 6-8liters/min. The comparatively high volumetric flow rate leads to betterdissipation of heat from the semiconductor wafers and from the immediatesurroundings thereof, that is to say polishing agent and polishing pad.

The invention enables higher flexibility with regard to the parametersand variations in process optimization, development and expansion of themethod. Using process-relevant measurement data, the invention allowsthe composition of the polishing agent to be varied.

Preferably, in a polishing sequence which requires three media, forexample, all three media are reused, topped up and/or replenished in themanner described.

Owing to the process-technological precautions, in particular by virtueof the fact that the polishing agent with the critical alkalinecomponents is continuously circulated, topped up and replenished, theingress of metal contamination in the bulk of the semiconductor wafercan be reduced. Critical metals are copper and nickel, in particular.

The polishing agent recycling also makes it economic to use particularlypure alkaline media such as TMAH, which further reduces the metalcontamination.

Finally, the diffusion rates of the metal ions can be reduced by thepolishing agent circuit being cooled.

1. A method for polishing a plurality of semiconductor waferscomprising: providing a polishing pad containing an abrasive substancebonded in the polishing pad; providing an alkaline polishing agent at avolumetric flowrate greater than or equal to 5 liters/min.; polishingthe plurality of semiconductor wafers using the polishing pad; andcirculating the polishing agent in a polishing agent circuit during thepolishing.
 2. The method as recited in claim 1, wherein the alkalinepolishing agent is ultrapure water containing at least one of sodiumcarbonate, potassium carbonate, sodium hydroxide, potassium hydroxide,ammonium hydroxide and tetramethylammonium hydroxide.
 3. The method asrecited in claim 1, wherein the alkaline polishing agent includes aslurry containing at least one abrasive selected from the groupconsisting of oxides of aluminum, cerium and silicon.
 4. The method asrecited in claim 2, wherein the polishing agent has a pH value in arange of 10 to
 12. 5. The method as recited in claim 1, wherein thepolishing agent includes colloidally dispersed silica and TMAH.
 6. Themethod as recited in claim 1, wherein the polishing pad includes atleast one abrasive selected from the group consisting of particles ofsilicon carbide, boron nitride and diamond and of oxides of cerium,aluminum, silicon and zirconium, and wherein the particles have anaverage particle size greater than or equal to 0.1 μm and less than orequal to 1.0 μm.
 7. The method as recited in claim 1, wherein thecirculating includes cooling the polishing agent during the circulating.8. The method as recited in claim 1, further comprising replenishing adefined amount or concentration of the polishing agent in a mannerregulated according to a chemical consumption.
 9. The method as recitedin claim 1, further comprising discharging a defined amount orconcentration of used polishing agent and replacing the discharged usedpolishing agent with unused polishing agent.
 10. The method as recitedin claim 1, further comprising collecting a first defined amount orconcentration of used polishing agent in a holding container, reusingthe first defined amount or concentration of the used polishing agentand replacing a second defined amount or concentration of the usedpolishing agent with a new polishing agent.